As this type of a damper there has been proposed a damper comprising a hydraulic damper and an actuator for imparting a propelling force to a piston rod of the hydraulic damper, as is disclosed in Japanese Patent Laid-Open Publication No. 2001-180244. According to this proposal, a rod of a hydraulic damper is formed in a tubular shape, a female screw portion is formed on an inner periphery side of the rod, a shaft connected at one end thereof to a rotor of a motor and at an opposite end thereof to a male screw member threadably engaged with the female screw portion of the rod is inserted through the rod of the hydraulic damper, and a piston rod of the hydraulic damper is constituted by the shaft and the rod.
According to the above proposal, force which is developed at the time of moving the shaft and the rod relatively axially by the motor to extend and retract the piston rod is added to a damping force developed in the hydraulic damper, that is, the force in question is acted additionally on the damping force of the hydraulic damper by converting the torque of the motor into force acting in the direction of the relative movement between the shaft and the rod, whereby it is intended to damp oscillation.
The damper disclosed in Japanese Patent Laid-Open Publication No. H08 (1996)-197931 is made up of a coil spring which resiliently supports a vehicle body side, i.e. a sprung member side of a vehicle, a screw shaft engaged threadably and rotatably with a ball screw nut which is connected to an axle, i.e. an unsprung member side of the vehicle, a motor connected to one end of the screw shaft and supported resiliently on the sprung member side by interposition between a pair of springs, and a hydraulic damper fixed to the vehicle body side to damp vertical oscillation of the motor. With rotational torque which the motor generates, a relative movement between the vehicle body and the axle is controlled actively.